The present invention relates to magnetic sound devices and more particularly to an electronic circuit arrangement for the quick braking of the drive motor of the sound recording medium transport of such a device.
Magnetic sound devices are usually provided with means actuated by a so called "rapid stop" or "pause" switch for short term interruption of the sound recording medium transport. In some tape devices, when the switch is actuated, the rubber pressure roll is lifted from the transport shaft, while the wind-off reel is simultaneously braked. With certain disc devices, the disc platter itself is braked or the transducer head is lifted from the disc. In all cases, a mechanical activity is required which can, of course, be easily executed by pressing a key associated with the switch or by using an electromagnet if the pause function is to be remotely controlled (i.e., through a microphone or foot switch).
Alternately, if an electromagnet cannot be used (e.g., for reasons of cost or space requirements) the current circuit of the drive motor for the transport must be opened or closed respectively as required to stop or start the motor. As a result, periods of continuously changing speed of the sound recording medium arise during acceleration of the motor from zero velocity to its operating speed and during its deceleration from its operating speed down to zero. During these periods of acceleration and deceleration, very noticeable disturbances are generated which obviously must be kept as small as possible. This can be done relatively easily and simply for motor start-up by properly designing the motor and speed control circuit. On the other hand, the deceleration time of the motor depends on the kinetic energy of the inertial mass and on friction and thus poses a special problem.
Heretofore, the drive motor of such devices have been braked electrically, for example by switching off the motor supply voltage while it still rotates and switching in a load resistance so that the motor works as a generator across the resistance. The braking effect becomes even greater when the motor is excited in the opposite direction after switch off. To this end, German Pat. DT-OS No. 2.126.891 suggests that a capacitor be charged while the drive motor is running and that this condenser be switched to the motor in reverse polarity when the supply voltage is switched off from the motor. The problem with this arrangement is that the capacitor must supply the entire braking energy. As a result, it must be relatively large in capacity as well as physical size requiring a device that is not only expensive, but oftentimes cannot be housed within the end product (e.g., small pocket dictating devices). In addition, while sound disturbances of the type referred to previously during starting and stopping are, indeed, considerably shortened in time with such prior art devices, they are not effectively removed.
In view of the above, the principal object of the present invention is to provide an electronic circuit arrangement for the rapid stopping of the drive motor of a magnetic sound device without using electromagnets, large capacitors or other similar, expensive and space consuming elements.
A further object of the present invention is to provide such a circuit which effectively silences the audio frequency voltages during the start-up and braking processes of the drive motor.